1. Field of the Invention
The present invention relates to a color cathode ray tube, and more particularly, to a color cathode ray tube capable of improving characteristics of luminance attenuation and explosion-proof through improvement of a shape of a panel.
2. Discussion of the Related Art
The structure of a general mask stretching-type color cathode ray tube is shown in FIG. 1.
Referring to FIG. 1, a vacuum envelope consisting of a rectangular panel 20 located on its front surface, a funnel 12 located on a rear surface of the panel 20, and a neck 6 extended from a rear end of the funnel 12 is sealed in a high vacuum pressure of about 10−7 Torr to secure smooth interlaced scanning of electron beams therein. An electron gun 8 is provided in the neck 6 to emit the electron beams 2 of red, green, and blue. A three-color (red, green, and blue) phosphor screen 16 and a color selection tension mask 18 are stretched in a vertical direction with respect to the cathode ray tube on an inner surface of the panel by a frame 15. The electron beams emitted from the electron gun 8 are controlled by a deflection yoke 4, and then are emitted onto a phosphor screen 16 to form an image.
According to the assembled structure of the tension mask 18 and the frame 15, as shown in FIG. 2, the tension mask 18 with electron beam passing apertures 18a of a grill or stripe type is welded at both ends of a long side to the frame 15, and is applied with tension in a direction parallel to the grill, i.e., vertical direction, by compression reacting force of the frame 15. The tension mask 18 is formed in a shape of straight line when viewing from a vertical direction, while the tension mask has a desired radius of a curvature, Rm, to have a convex shape with respect to an axis of a cathode ray tube, similar to an inner curvature of the panel 20, when viewing from a horizontal direction. The electron beam passing apertures 18a formed on the tension mask 18 have a desired pitch in a horizontal direction.
The panel 20 attached to the front surface of the vacuum envelope 1, the inside of which is maintained in a vacuum condition to secure smooth interlaced scanning of electron beams, is shown in FIGS. 3a and 3b. 
The panel 20 having a generally rectangular shape includes an effective surface 22 on which the phosphor screen 16 is formed, a long side 24 formed in a horizontal direction at both ends of a vertical axis, a short side 26 formed in a vertical direction at both ends of a horizontal axis, and a corner 28 forming both ends of a diagonal axis. The sides and corner are bent toward a rear of the tube axis from an edge of the effective surface to form a skirt 29.
FIG. 4 shows a shape of the effective surface 22. A curvature radius of an outer surface, Ro, of the effective surface seems to be a flat surface when viewing visually, while a curvature radius of an inner surface thereof is formed in a non-spherical shape. Specifically, the curvature radius of the inner surface may be represented by three curvatures, i.e., a vertical inner curvature radius Riv, a horizontal inner curvature radius Rih, and a diagonal inner curvature radius Rid.
The above three curvature radiuses of the panel for the conventional mask stretching-type flat color cathode ray tube is generally manufactured according to a condition of Riv>Rid>Rih, or Riv≅Rid>Rih. In addition, a ratio of Riv/Rid has a range of 1.00 to 1.20, and a ratio of Riv/Rih has a range of 0.36 to 1.5. Wedge amount (a ratio of a thickness of an diagonal end of the effective surface of the panel to a thickness of a center portion of the panel, i.e., Tc/CFT) is in the order of about 1.3.
The inner curvature Ri of the panel for the conventional mask stretching-type flat color cathode ray tube constructed described above is determined as follows:
FIG. 5a shows a geometrical relationship of a conventional formed mask-type flat color cathode ray tube, and FIG. 5b shows a geometrical relationship between the electron beams and the panel and mask with respect to the conventional mask stretching-type flat color cathode ray tube.
Referring to FIG. 5a showing the conventional formed mask-type flat color cathode ray tube, in order to maintain a value of beam arrangement as ‘1’ (the value of the beam arrangement indicates the order of constantly arranging a space to an adjacent electron beam after the electron beam 2 passes through the apertures of the mask 19 and reaches the inner surface of the panel), the geometrical relationship among the inner curvature Ri′ of the panel, the curvature Rm of the formed mask, and the electron beam is represented as follows:   GR  ∝            S      ×      Q              Ph      ×      L                      where, GR is beam arrangement between peripheral electron beams, S is a distance between a center electron beam and peripheral electron beams on a deflecting center, Q is a distance between the inner surface of the panel and the mask on a pathway of the electron beam, and Ph is a distance between the passing aperture of the mask and a peripheral passing space at a position to which the electron beam reaches.        
In the above relationships, on the basis that the electron beam is emitted onto the center of the panel, the more the electron beam is emitted in a peripheral direction, the more increasing the value L is. Since it is changed in a type of Lo (a distance from the center of the panel)<L′ (a distance from the peripheral portion of the panel), the value Q is increased as it goes toward the peripheral portion to maintain a condition of GR=1. Therefore, a condition of Qo (a distance form the center portion of the panel)<Q′ (a distance from the peripheral portion of the panel) is necessary. In case of the formed mask-type flat color cathode ray tube, the increase of the value Q required in the peripheral portion can be adapted by transforming the shape of the mask. Accordingly, when determining the inner of curvature of the panel, it is possible of design it, in view of a floating effect of the image according to the thickness of the panel and a mechanical strength under the vacuum state.
The structure of vertical, horizontal, and diagonal curvature is satisfied with the condition of Rid>Rih>RiV, that is favorable for the structure of panel vacuum stress.
According to the mask stretching-type flat color cathode ray tube shown in FIG. 5, each value Q of the center portion and peripheral portion of the panel is under a condition Qo (center portion)>Q′ (peripheral portion; 6 and 12-o'clock directions), which is contrary to the results of the formed mask-type flat color cathode ray tube, depending upon a mode of the tension mask 18 the mask of which is vertically stretched every section. Therefore, as it goes toward the peripheral portion (6 and 12-o'clock directions), the value GR is lower than 1. Contrary to the formed mask 19, since the vertical curvature of the tension mask 18 is infinite, there is technical difficulty that it does not meet the variation of the value Q to maintain GR=1.
In the mask stretching-type flat color cathode ray tube, since it does not meet the variation of the value Q using the curvature of the mask, the vertical curvature radius Riv of the panel of FIG. 4 is formed larger than the horizontal curvature radius Rih and the diagonal curvature radius Rid. Specifically, the increase of required value Q is met by increasing the value Riv in a more flat direction. After all, the structure of curvature radius of each axis consists of a condition of Riv>Rid>Rih or Riv≅Rid>Rih.
The value GR required for maintaining the quality of picture optimally has to satisfy a range of 1∀0.03. In case that the structure of the inner curvature radius of the panel of each axis is formed by the structure of Rid>Rih>Riv which is the condition of the formed mask-type flat color cathode ray tube, the value GR is below about 0.80, thereby deteriorating the picture in order of not displaying the basic picture of the cathode ray tube.
The structure of the mask stretching-type flat color cathode ray tube has the structure of Riv>Rid>Rih or Riv≅Rid>Rih. In the structure that the vertical inner curvature is more flat than the horizontal or diagonal curvature, the thickness of the vertical panel glass is thinner than that of the diagonal or horizontal panel glass on the basis of the same wedge amount (a ratio of a thickness of a diagonal end of the effective surface of the panel to a thickness of a center portion of the panel).
The vacuum stress is increased by such the structure when evacuating the vacuum envelope of the cathode ray tube, thereby raising a safety problem. Specifically, in case of evacuating the vacuum envelope 1 consisting of the panel 20 and the funnel 12, strong tension stress is happened at the panel 20, the situation shown in FIG. 6.
FIG. 6 shows distortion of the vacuum envelope when evacuating the vacuum envelope. When evacuating the vacuum envelope, the effective surface 22 of the panel is distorted inwardly on the basis of the center of the panel 20, and the skirt 29 of the panel is distorted outwardly. According to the above distortion, the edge of the effective surface 22 with a flat outer surface is applied with the strong tension stress. The vertical end Ev of the effective surface is applied with the maximum tension stress. According to the structure of the conventional panel, the inner curvature radius is increased to meet the increase of demanded value Q. The reduced vertical thickness of the glass is coupled to the portion from which the maximum tension stress is generated, so that the stress is increased to the extreme extent, thereby causing the explosion-proof characteristic to be reduced and so generating the safety problem.
For example, in case of the mask stretching-type flat color cathode ray tube of 32 voltages, the tension stress of above about 12 Mpa is generated, thereby exceeding a tolerance limit of tension stress, 10 Mpa. In order to solve the problem, the conventional cathode ray tube increases the thickness of the outer surface of the panel by a predetermined degree a, as shown in FIG. 5b, to suppress the generation of stress at the effective surface. However, the method increases extremely the thickness of the center portion of the panel in relation to the formed mask-type flat color cathode ray tube.
For example, in case of the formed mask-type flat color cathode ray tube of 32 voltages, the thickness of the center portion of the panel is 15 t, while in case of the mask stretching-type flat color cathode ray tube the thickness of the center portion of the panel is 21.5 t, thereby increasing the thickness of about 43 percentages.
In addition, the increased thickness of the panel causes a light transmittance to be reduced, thereby deteriorating the luminance characteristic. Breakage is increased during an annealing process of the cathode ray tube, and thermal process index is reduced. The increased weight of the panel causes materials and manufacturing costs to be increased.